The present invention relates to a variable valve mechanism for an engine.
In an engine, improvement in both output and fuel efficiency is realized by assembling a cam-switching variable valve device onto a valve operating system. The variable valve device is a device for switching the opening/closing timing of a valve and the lifting amount thereof by switching cams. In the variable valve device, a plurality of types of cams different in cam profile from each other are provided side by side on a camshaft. And, by selectively switching rocker arms that follow these cams, displacement of the cam to be transmitted to the valve is changed.
In such a case that an engine is mounted in an automobile, with the aim of making the cylinder head itself compact while avoiding engine components, the camshaft is disposed obliquely above the rocker shaft, for example. Such a variable valve device assembled on an engine has been disclosed in, for example, JP-A-2005-105953. The variable valve device for an engine disclosed in JP-A-2005-105953 includes a low-speed rocker arm linked to a valve and a high-speed rocker arm combined with the low-speed rocker arm. Both rocker arms are attached so as to be swingable while being fitted to the outside of the rocker shaft, and structured so that a cam displacement is transmitted from the high-speed rocker arm to the low-speed rocker arm.
In the switching mechanism disclosed in JP-A-2005-105953, it can be said that the low-speed rocker arm can be restricted from displacement to both sides in a rocker shaft axis direction, while the high-speed rocker arm is restricted from displacement in the rocker shaft axis direction in only one direction to hit against the low-speed rocker arm. In detail, as shown in FIG. 2 of JP-A-2005-105953, the high-speed rocker arm is structured to be restricted from displacement in a direction of arrow D as a result of hitting against the low-speed rocker arm but to be displaceable in a direction of arrow C opposite to the arrow D direction. Therefore, in the switching mechanism, for restricting the high-speed rocker arm from displacement in the arrow C direction, it is necessary to position the high-speed rocker arm by the cylinder head by, for example, making the high-speed rocker arm abut against a part of the cylinder head. However, when the rocker arm abuts against the cylinder head, friction occurs at a contact part therebetween at the time of rocker arm operation, and thus a smooth operation of the rocker arm is hindered.
Furthermore, the switching mechanism has a structure where an arm of the high-speed rocker arm is located at a back face side of a boss portion of the low-speed rocker arm, and the cylinder head is increased in width for that. Alternatively, for forming a bearing portion of the rocker shaft in the cylinder head, a rod-like tool is inserted inside the cylinder head and cutting is performed, and in order to secure a space for inserting the tool, it is also necessary to increase the cylinder head in size. Accordingly, it can be said that, for applying the variable valve device of JP-A-2005-105953, there is a tendency that the engine as a whole is increased in size.